Summary Developing males are broadly vulnerable, making them more susceptible to early onset neuropsychiatric and neurological disorders and more severely affected by neonatal brain injury. Clues to the origins of heightened male vulnerability can be found in the mechanisms of masculinization of brain and behavior. Working with the laboratory rat we have discovered surprising origins of sex differences in the brain that begin with non-neuronal immune cells and inflammatory signaling molecules. In particular we find that microglia, the brains innate immune system, are more numerous and more activated in regions of male brain than female and that these cells are a dominant source of the prostaglandin, PGE2, which is also elevated in males compared to females. Mast cells, another non-neuronal immune cell, are also found in greater numbers and in a more activated state in males. Histamine released from mast cells stimulates PGE2 production by neighboring microglia and promotes the formation and stabilization of dendritic spine synapses. The result is sexually differentiated synaptic patterning that regulates adult sex-typic behavior. We further determined that immune response genes are epigenetically repressed in female brains compared to males, due to greater DNA methylation. Pilot data connects neuroinflammation and epigenetics with the observation that either PGE2 or mast cell degranulation can inhibit DNA methylating enzymes. Specific Aim 1 will measure the impact of neuroinflammation on the epigenome in developing brain while Specific Aim 2 takes the opposite tact and will test the effect of epigenome regulation on neuroinflammation in developing brain. Ultimately the question is why normal healthy males have higher neuroinflammation during development and here we propose the novel PREMISE: Maternal immune response directed at male fetuses increases their vulnerability. In Specific Aim 3 we will utilize a female dam with a male immune system via bone marrow transplantation to test the impact of the sex of the maternal immune system on the sexual differentiation of her offspring. Chimeric females will be generated using GFP-expressing stem cells to allow for tracking of maternal immune cells into the placenta and fetal compartment, including the fetal brain. Maternal Immune Activation (MIA) will be employed to assess for pathology. For each aim we will conduct experiments that measure inflammatory molecules, microglia and mast cell activation, DNMT activity and DNA methylation, the transcriptome using nanoString quantification of mRNA and juvenile and adult behaviors influenced by sex.